The present invention relates to a semiconductor laser controller for controlling a light output of a semiconductor laser used as a light source in a laser printer, an optical disk apparatus, an optical communication device, etc.
Recently, an optical disk apparatus of a write-recordable postscript type (WO type) has been used to perform reproduction of information and further writing and recording operations thereof. Further, there is a disk apparatus of a magnetic optical type (MO type) which has an erasing function as well as the writing function and can perform a rewriting operation. In such optical apparatuses, with respect to the light intensity of a laser beam emitted from a semiconductor laser, two kinds of light intensities for reproduction and record are set in the case of the optical disk apparatus of the WO type. In the case of the optical disk apparatus of the MO type, the light intensity for erasion is further added to the above two light intensities so that three kinds of light intensities are set. In any case, the stabilization of the light intensities of the laser beam is indispensable to the stable reproducing, recording and erasing operations.
In a system for controlling the light intensity for reproduction of the laser beam of the semiconductor laser, a normal operational amplifier is used as an amplifier and a comparator. Since such an operational amplifier is used, when power for reproduction is varied, this variation cannot be restrained for a time about a minimum pulse width of reproduced data. As a result, there are cases in which focusing and tracking servo controls are not suitably performed and an error in reproduced data is caused. This is because a return light with respect to the semiconductor laser exerts a bad influence on the power control of this semiconductor laser and this influence increases as the laser power is low as that for reproduction. At the reproducing time, the reflected light is varied by a reflected light from a really data-existing portion and a reflected light from a data non-existing portion so that the degrees of the above influence are different from each other.
As a system for reducing the influence of such a return light, there is a system using a 1/4 waveform plate. Namely, in the optical disk apparatus of the WO type, when a linearly polarized light with respect to the optical disk from a beam splitter is converted to a circularly polarized light by the 1/4 waveform plate, the disk reflected light is returned as the circularly polarized light to the 1/4 waveform plate. When this reflected light passes through the 1/4 waveform plate, the reflected light becomes a linearly polarized light having a phase different by 90.degree. from that of an incident light. This reflected light is then incident to the beam splitter again so that the reflected light is not returned onto the side of the semiconductor laser, but is directed to only a detecting system. However, in the optical disk apparatus of the MO type, the 1/4 waveform plate cannot be inserted into the apparatus. This is because it is necessary that the spot light beam irradiated onto an optically magnetic disk face is a linearly polarized light to reproduce a signal by the direction of the polarized light in the optical disk apparatus of the MO type. Accordingly, when the optical disk apparatus is constructed to be of both the WO and MO types, it is necessary to set the 1/4 waveform plate to be inserted at the using time of the apparatus of the WO type, and detach the 1/4 waveform plate at the using time of the apparatus of the MO type. Therefore, as the disk apparatus, it is necessary to dispose parts for moving and operating the 1/4 waveform plate, a drive circuit therefor, etc. Such a construction is disadvantageous with respect to reliability, cost, outer size, weight, etc. Such a situation is similarly caused in a reproduction power control in a system for controlling the respective light intensities for reproduction, record and erasion.
In this control system, to control the operation of the semiconductor laser and stabilize its light output, it is necessary to dispose two independent control circuits composed of a semiconductor laser drive control system for only reproduction and a semiconductor laser drive control system for only record and erasion, thereby increasing the number of circuits and parts. Further, at the really recording time, the control of a constant electric current is performed and is different from that at the control time of the recording power. Therefore, the restriction of the variation of the recording power at the really recording time becomes insufficient.
Further, conventionally, a laser diode drive unit cannot be stably operated at a high speed with respect to a low frequency caused by the change in temperature, etc.
The semiconductor laser is very compact and can be directly modulated by a drive electric current at a high speed. Therefore, recently, the semiconductor laser has been widely used as a light source in the optical disk apparatus, a laser printer, etc.
However, the drive electric current and light output characteristics of the semiconductor laser are greatly changed by temperature. This change causes a problem when the light intensity of the semiconductor laser is set to a desired value. Various kinds of APC (Automatic Power Control) circuits have been proposed to solve this problem and use the advantages of the semiconductor laser.
These APC circuits are generally divided into the following three systems.
(1) In a first system, the light output of the semiconductor laser is monitored by a light-receiving element. A photoelectric negative feedback loop for controlling a forward electric current of the semiconductor laser at any time is disposed such that a signal proportional to a light-receiving electric current (proportional to the light output of the semiconductor laser) generated in this light-receiving element is equal to a command signal indicative of a luminous level. In this system, the light output of the semiconductor laser is controlled to be a desired value by this photoelectric negative feedback loop.
(2) In a second system, the light output of the semiconductor laser is monitored by the light-receiving element in a power setting period. The forward electric current of the semiconductor laser is controlled such that the signal proportional to the light-receiving electric current (proportional to the light output of the semiconductor laser) generated in this light-receiving element is equal to the luminous level command signal. In a period except for the power setting period, the forward electric current value of the semiconductor laser set in the power setting period is held to control the light output of the semiconductor laser to be a desired value. In the period except for the power setting period, the forward electric current of the semiconductor laser is modulated by information with the forward electric current value of the semiconductor laser set in the power setting period as a reference, thereby superimposing the information on the light output of the semiconductor laser.
(3) In a third system, the temperature of the semiconductor laser is measured. The light output of the semiconductor laser is controlled to be a desired value by controlling the forward electric current of the semiconductor laser by the measured temperature, or controlling the temperature of the semiconductor laser to be constant.
The system (1) is desirable to set the light output of the semiconductor laser to be the desired value. However, a limit is caused with respect to control speed by limits of the operating speed of the light-receiving element, an operating speed, etc., of an amplifying element constituting the photoelectric negative feedback loop. For example, in consideration of a cross frequency in an open loop of the photoelectric negative feedback loop as a reference of this control speed, the step responsive characteristics of the light output of the semiconductor laser can be approximately provided as follows when this cross frequency is set to f.sub.0. EQU P.sub.out =P.sub.0 {1-exp (-2.pi.f.sub.0 t)}
where P.sub.out is the light output of the semiconductor laser, P.sub.0 is the light intensity set in the semiconductor laser, and t is time.
In many cases in which the semiconductor laser is used, it is necessary to set to a predetermined value the entire light amount (an integral value .intg.P.sub.out of the light output) from a time immediately after the light output of the semiconductor laser is changed until a set time .tau..sub.0 has passed. This integral value is provided as follows. ##EQU1##
If the time .tau..sub.0 =50 ns and an allowable range of the error is set to 0.4%, the cross frequency f.sub.0 &gt;800 MHZ must be set. It is very difficult to set the cross frequency to such a value.
In the system (2), the above problems with respect to the system (1) are not caused and it is possible to modulate the semiconductor laser at a high speed. Accordingly, the system (2) is used in many cases. However, in this system (2), the light output of the semiconductor laser is not controlled at any time so that the light amount of the semiconductor laser is easily varied by a disturbance, etc. As the disturbance, DO loop characteristics of the semiconductor laser for example are considered. An error about several % is easily caused by the DO loop characteristics with respect to the light amount of the semiconductor laser. A compensating method by combining a thermal time constant of the semiconductor laser and the frequency characteristics of the semiconductor laser drive electric current is proposed as a trial for restricting the DO loop characteristics of the semiconductor laser. However, in such a method, there is dispersion every semiconductor laser with respect to the thermal time constant of the semiconductor laser. Further, this thermal time constant is different by the environment of the semiconductor laser.
Further, the light amount of the semiconductor laser is varied by the influence of the return light in the optical disk apparatus, etc.